Creating a 3D printable terrain model with blender
Starting from the displaced plane
Apply the displace modifier
Enter edit mode Tab
Select all A
Place yourself in front (NumPad1) ortho view (NumPad5 if needed)
Extrude E then Z 
Now scale along Z to zero to flatten the base : S Z then 0


Note: all keystrokes are not visible in the gif as 500 subdivisions is a bit too much on my computer. But the keys are written above (and you can see the operations on the bottom left of the screen)